This invention relates generally to techniques for removal of particulate contaminants during semiconductor processing and, more particularly, to methods for removing contaminant particles that may accumulate in a plasma reactor chamber.
In the manufacture of microelectronic devices, particulate contamination has become of increasing concern because semiconductor devices are being packed with ever-increasing densities on semiconductor chips. Usually, multiple chips are formed from a single larger semiconductor wafer, six to eight inches (15-20 cm) in diameter, but overall chip dimensions have been increasing in recent years and some integrated circuits may occupy areas comparable with a whole wafer. Concurrently, there has been a continuing trend toward smaller device geometries, i.e. higher device densities. Obviously, use of a smaller device geometry means that smaller contaminant particles may cause defects in a circuit. The presence of particles during deposition or etching of thin films can cause voids, dislocations or short-circuits, which adversely affect performance and reliability of the devices. Moreover, larger overall device dimensions result in a lower percentage yield of satisfactory devices.
Particle contamination has been significantly reduced by improving the quality of clean rooms and automated equipment designed to handle semiconductor wafers, and by improving techniques to clean the wafer surfaces. However, many particles are generated inside wafer processing chambers themselves. Possible sources of contamination include processing materials, interior walls of processing chambers, and mechanical wear of the automated wafer handling equipment. More specifically, in processing steps that use a plasma many chemical "fragments" of various kinds are generated from the processing gases, including ions, electrons and radicals. The fragments can combine to form generally slightly negatively charged particles, which may ultimately contaminate a wafer being processed. In addition, various materials, such as polymers, are coated onto the process chamber walls during plasma processing. Mechanical and thermal stresses may cause these materials to fracture and dislodge from the walls, generating additional contaminant particles. Other sources of contaminants are oil from vacuum pumps, and particles generated within the processing chambers during wafer transfer operations.
In plasma processing, contaminant particles typically become trapped in the chamber, between a plasma sheath adjacent to the wafer and a plasma glow region. The particles pose a significant risk of wafer contamination, especially at the end of plasma processing, when radio-frequency (rf) power that sustains the plasma is switched off. For many plasma processes, a focus ring is disposed above and at the circumference of the wafer, to enhance uniformity of processing by controlling the flow of active plasma species to the wafer, such as during a plasma etch process. The focus ring, and an associated wafer clamping mechanism, effectively preclude removal of the trapped particles by gas flow, unless the pressure in the chamber is reduced to an unacceptably low level. It will be appreciated that there is still a need to provide a reliable and inexpensive process to remove such particles from the wafer processing chamber. The present invention fulfills this need.